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Fabrication of New Synthetic Routes for Functionalised SiMCM-41 Materials as Effective Adsorbents for Water Remediation Haribandhu Chaudhuri, Subhajit Dash, and Ashis Sarkar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02241 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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Industrial & Engineering Chemistry Research

Fabrication of New Synthetic Routes for Functionalised Si-MCM-41 Materials as Effective Adsorbents for Water Remediation

Haribandhu Chaudhuri, Subhajit Dash, and Ashis Sarkar* Organic Materials Research Laboratory, Department of Applied Chemistry, Indian School of Mines, Dhanbad, Jharkhand-826004, India. *Corresponding author: Ashis Sarkar, E-mail: [email protected]; Tel. +91 9430335255; Fax: +91 326-2307772. Abstract: The removal of toxic dyes (Methyl violet, MV; Congo red, CR; and Malachite green, MG respectively) from water using aspartic acid (APA), polyethyleneimine (PEI), and βcyclodextrin (β-CD) functionalised Si-MCM-41 developed through new facile synthetic routes is reported here. The successful functionalisations of Si-MCM-41 were examined by means of TGA, solid state

13

C CP MAS NMR spectra, FTIR, XRD, nitrogen sorption,

FESEM, and TEM. Beside this, the sorption behavior of those sorbents using sorbates is determined by considering some factors like the pH of the medium, contact time, temperature of the medium, concentration of the dye medium, sorbent dose, and agitation speed. The kinetics data fitted in accordance with pseudo-second-order model. It is noticed that Langmuir equation proposes an accurate description of these sorption data, implying that monolayer adsorption took place in all sorption processes. The proficient sorption capacities of these adsorbents (qm: 374.89 mg g-1 for MV on Si-MCM-41-APA, qm: 458.72 mg g-1 for CR on Si-MCM-41-PEI, and qm: 561.45 mg g-1 for MG on Si-MCM-41-β-CD) can be described from the view point of both hydrogen bonding and electrostatic interactions

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between the sorbents and sorbates. Moreover, both sorbents and sorbates can be recycled and reused after regeneration process. Keywords: Functionalised Si-MCM-41; new synthetic routes; adsorbents; toxic dyes; water remediation. 1. Introduction: Over the last few years, mesoporous silica materials (MSMs) have received a significant attention due to their special characteristics of high surface area, uniform pore radius, and large pore volume, and have been flourished for different applications like sorption[1], catalysis[2], drug delivery[3], energy storage[4] etc. As MSMs are thermally stable, huge efforts have been made for the development of functionalised MSMs with different active organic sites. Functionalisation of different MSMs with useful organic moieties on the mesostructured silica walls makes huge scope of research.[5-11] Several adsorbents with useful binding sites have been developed through grafting or functionalisation procedures for efficient water purification.[12-30] Amino acid, branched polyamine, β-cyclodextrin and its derivative functionalised MSMs were incorporated into the mesopores and the resulting materials were used as sorbents for water purification.[8,

13, 28, 31-34]

Though uncountable

experiments have already been done in this area, but still it is very much important to fabricate more advanced adsorbents with new procedures for water remediation.[35-41] Recently, we have synthesised amino- or carboxylic group containing SBA-15 after activation of surface Si-OH groups using concentrated acid. The functionalised solids showed proficient sorption of toxic dyes from aqueous solution in short time.[42] As an extension of our work, here we report APA, PEI, and β-CD functionalised Si-MCM-41 adsorbents via facile and efficient synthetic routes solely based on the formation of siloxyderivative of npropylcabanoyl chloride (SPCC) intermediate using low cost reagents. As demonstrated in


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scheme 1, APA, PEI, and monodiamino-β-cyclodextrin (β-CD-NH2) were functionalised onto SPCC, obtaining Si-MCM-41-APA, Si-MCM-41-PEI, and Si-MCM-41-β-CD respectively. The used toxic dyes such as CR, MG, and CV can be proficiently adsorbed from water owing to the existence of numerous active groups on the surface of those sorbents. In this respect, CR, a potential hazard of bioaccumulation and cause allergic problems, is usually known for the metabolization of benzidine. So, it’s choice as adsorbate has been considered for many reasons like its limited stability and biodegradability in sunlight, complex chemical structure as well as high solubility in water.[43-45] CV, a bacteriostatic agent and skin disinfectant, is used in paints, printing ink cotton, and silk. Moreover, it is harmful to the eyes of mammals.[46] MG, a well-known disinfectant and used in aquaculture, can incorporate obnoxious color to water bodies with reduction in sunlight penetration.[47-49] The sorption behaviours were also examined with respect to the pH of the medium, contact time, temperature of the medium, sorbent dose, concentration of dye medium and agitation speed to optimize the adsorption conditions. The executed measurements indicate that those functionalised sorbents show swift rate of adsorption along with excellent adsorption efficacies which are ascribed due to great affinity of copious number of functional groups, enormous number of active sites, and huge surface area. In addition, adsorption characteristics of these sorbents were investigated by using various isotherm models like Langmuir, Freundlich, and Temkin. 2. Experimental Section: 2.1. Materials. Hexadecyltrimethylammonium bromide (CTAB), tetraethoxysilane (TEOS, ≥99%), triethanolamine (TEA, ≥99%), 3-(triethoxysilyl)-propionitrile (≥98%), β-cyclodextrin (β-CD), aspartic acid (APA, ≥99%), polyethyleneimine (PEI, ≥98%), p-toluenesulphonyl chloride (≥98%), and ethylenediamine (~98%) were procured from Sigma Aldrich. Acetonitrile (~98%), dimethylformamide (DMF, anhydrous, ~98%), toluene (anhydrous, 3 ACS Paragon Plus Environment


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~98%), benzene (anhydrous, ~98%), ethanol (EtOH, ≥ 99%), sulphuric acid (H2SO4, ~98%), hydrochloric acid (HCl, ~37%), and sodium hydroxide (NaOH) were purchased from Merck India. MV (~98 %, λmax: 581 nm), CR (~98 %, λmax: 499 nm) and MG (~98 %, λmax: 620 nm) were procured from Loba Chemie and their structures are presented in Figure S1 in supporting information, SI. 2.2. Syntheses. MSM was synthesised according to the procedure demonstrated earlier.[50] In a typical process, 128 mL of water (0.36 mol) and 20.8 g of a 25 wt% CTAB solution (0.786 mmol) were combined and stirred. 18 g of ethanol (0.015 mol) and 0.4 g of TEA (0.19 mmol) were added to the medium and stirred at 60°C for 60 min. Then, 14.6 mL of TEOS (3.25 mmol) was added to the homogeneous solution dropwise within 10 min under stirring. After that, the stirring was continued for 3 h until the solution turned milky white. The solid product was cooled at room temperature, filtered, washed with distilled water along with distilled ethanol and dried for 12 h under vacuum (yield~98 %). Finally, the solid was calcined at 550°C for 6 h. Before functionalisation, the synthesised calcined MSM was refluxed in concentrated HCl for 12 h. Thereafter, the resulting solid was repeatedly treated with distilled water until the eluent became neutral and dried by heating at 115°C for 24 h under vacuum. Carboxy functionalised MSM was synthesised following the method reported earlier.[51] Initially, 6 g of activated MSM and 3-(triethoxysilyl)-propionitrile (0.1 mole) in dry toluene (75 ml) were combined and stirred in round bottom flask at 120°C for 24 h, resulting in cyano functionalised MSM. Then, it was repeatedly treated with dry toluene and dried at 110°C under reduced pressure. The cyano functionalised MSM was refluxed with 90 ml of acid (sulphuric acid, 50 %) at 130°C for 12 h. The so formed carboxylic group containing MSM was washed extensively with distilled water to make it free from acid and dried at 110°C under reduced pressure.


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Thereafter, the solid was refluxed with thionyl chloride in presence of 90 ml of anhydrous benzene at 80°C for 6 h and the excess thionyl chloride was distilled off along with anhydrous benzene to form SPCC intermediate. Herein, this intermediate is crucial to produce Si-MCM-41-APA, Si-MCM-41-PEI, and Si-MCM-41-β-CD adsorbents. APA functionalised Si-MCM-41 was synthesised when 2 g of SPCC and 0.1 mole of APA (partially dissolved in freshly prepared sodium ethoxide) were mixed in 25 ml of dry benzene and stirred at 80°C for 12 h and the excess dry benzene was distilled off. The resulting solid was repeatedly washed with dry benzene and dried under reduced pressure. For the synthesis of PEI functionalised Si-MCM-41, 2 g of SPCC was refluxed with 0.1 mole of PEI in 25 ml of dry benzene at 45°C for 12 h. Then, the excess dry benzene was distilled off and the material was repeatedly washed with dry benzene and dried under reduced pressure. β-CD functionalised Si-MCM-41 was synthesised when 0.1 mole of β-CD-NH2 (prepared following a reported procedure[52]) was mixed in 2 g of SPCC in 25 ml of dry DMF at 90°C and refluxed for 12 h and the excess dry DMF was distilled off. Finally, the product was washed repeatedly with dry DMF and dried under reduced pressure. 2.3. Characterizations. Thermogravimetric analysis (TGA, NETZSCH STA-449f3, Jupiter) was recorded on a thermal analyser in the temperature range 30°C-900°C at a heating rate 5°C/min in nitrogen flow. Fourier transformed infrared spectroscopic (FTIR) analyses were performed using FTIR spectrometer (Model IR-Perkin Elmer, Spectrum 2000) using KBr pellet method. Powder X-ray diffraction (XRD) patterns were obtained using Thermal ARL X-ray diffractometer with CuKα radiation source. N2 sorption measurements were done at 77 K with Nova 3200e (Quantachrome, USA). Field emission scanning electron microscopic analyses (FESEM, Supra 55, Zeiss, Germany) and transmission electron microscopic analyses (TEM, JEM-2100, JEOL, Japan) were employed to get a brief idea about the pre and post adsorbed surfaces of those sorbents. Solid state 13C cross-polarization (CP) magic angle



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spinning (MAS) NMR spectra were analysed using JEOL ECA400 MHz instrument operated with 4 mm CD/MAS prove at room temperature. The zeta potentials were measured using a Zetasizer Nano-ZS90 (Malvern, UK). Measurement range is 3.8 nm-100 μm having the electrophoretic light scattering feature. 2.4. Adsorption measurements. Adsorption of those toxic dyes using functionalised MSMs was done using an orbital shaker (Rivotek, Kolkata, India) and the absorbance was recorded using a UV–Vis spectrophotometer (Shimadzu, Japan; Model: UV 1800). Specifically, the pH of the medium, contact time, temperature of the medium, sorbent dose, concentration of dye medium and agitation speed were performed according to the procedure depicted in the literature.[53, 55] The percent of dye sorption was evaluated following eq 1.[53-55] % 𝐴𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 =

𝐶0 − 𝐶𝑒 × 100 𝐶0

(1)

The dye sorption at equilibrium was calculated following eq 2.[53-55]

𝑞𝑒 = (𝐶0 − 𝐶𝑒 ) ×

𝑉 𝑊

(2)

Where equilibrium capacity of dye (mg g-1) solution denoted as qe, initial and equilibrium concentration of sorbate solutions (mg L-1) are represented as C0 and Ce respectively. V denotes the volume of the sorbate solution (L) used and weight of the sorbent (g) is expressed as W. Here, the results of adsorption are the arithmetic means of three studies. 2.5. Desorption measurements. The reusability of functionalised MSMs was examined in four consecutive sorption cycles. The desorption study was done following the procedure reported earlier.[50, 52] Multiple pH solutions (pH: 2, 7 and 10) were employed to determine the maximum regeneration efficiency of dye from those adsorbents (Desorption conditions for MV: concentration of dye solution, 100 ppm; adsorbent, 15 mg; temperature, 318 K; and contact time, 30 min; agitation speed, 110 rpm: Desorption conditions for CR: concentration of dye solution, 100 ppm; adsorbent, 8 mg; temperature, 313 K; and contact time, 50 min;


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agitation speed, 100 rpm: Desorption conditions for MG: concentration of dye solution, 100 ppm; adsorbent, 10 mg; temperature, 308 K; and contact time, 40 min; agitation speed, 110 rpm. The % desorption was calculated using eq. 3.[53-55]

% Desorption =

Concentration desorbed (mg/L) × 100 Concentration adsorbed (mg/L)

(3)

3. Results and Discussion: 3.1. Characterizations of Pre and Post Adsorped Materials. From Figure 1(a), it may be noted that weight changes of all the above mentioned samples were monitored by TGA curves. Below 150°C, all the curves show a decreased weight loss due to the elimination of physicochemically attached water molecules of those materials. For Si-MCM-41-APA, 16.5% weight loss occurred between 150°C and 800°C, in which about 11.5% weight loss owing to the loss of organic groups incorporated during co-condensation reaction. Furthermore, for Si-MCM-41-PEI, about 17% reduction in weight observed between 150°C and 700°C, of which about 12% loss is attributed to the loss of organic components introduced by co-condensation. Beside this, for Si-MCM-41-β-CD, about 19.5% weight loss occurred between 175°C and 800°C, of which about 14.5% loss is mainly owing to organic part employed during co-condensation reaction. For all above three compounds, it can be demonstrated that functionalizations of concentrated HCl treated calcined MSM have been successfully done via the formation of SPCC intermediate. The successful surface functionalisation of concentrated HCl treated material by APA, PEI and β-CD was confirmed by solid state

13

C CP MAS NMR spectroscopic studies. Figure

S2(i) (SI) shows strong peaks at 8.2 ppm, 14.9 ppm, and 28.4 ppm which are ascribed to the characteristic peaks of the aliphatic carbons on the chain between APA and the adsorbent 13]

[8,

. The peak at 178.6 ppm implies the formation of –C=O of the formed urea bond and 7 ACS Paragon Plus Environment


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signals resonating between 60 ppm and 70 ppm in the spectrum are assigned to the incorporation of APA on Si-MCM-41. From Figure S2(ii) (SI), peaks at 8.4 ppm, 14.9 ppm, and 28.2 ppm which indicate the presence of aliphatic carbon chain of PEI on Si-MCM-41 [8]. A strong signal at 179.3 ppm is due to the presence of carbonyl carbon of urea bond. Beside this, spectrum of Si-MCM-41-β-CD (Figure S2(iii), SI) conveys the peaks at 8.5 ppm, 14.7 ppm, and 28.3 ppm which are ascribed due to the presence of aliphatic carbon chain between β-CD and the material

[31, 33]

. Signals resonating between 60 ppm and 120 ppm in the

spectrum are attributed to characteristic carbon peaks on the β-CD, and the peak at 178.8 ppm indicates the presence of carbonyl carbon of the urea bond. All these observations indicate successful implementation of APA, PEI, and β-CD on the activated surface of MSM. The FTIR spectra were collected for all pre and post adsorbed materials and shown in Figure 2(b). As shown in Figure 2(b), concentrated HCl treated calcined MSM exhibits a large band of silanol -OH stretching band at 3454 cm-1. The band at 1637 cm-1 is observed because of the existence of bending vibration in Si-OH

[56]

. Bands associated to asymmetric stretching

vibration Si-O-Si and Si-O bending in Si-OH have been observed at 1089 cm-1 and 987 cm-1 respectively

[57]

. Moreover, symmetric Si-O-Si stretching mode is noticed at 806 cm-1. Si-

MCM-41-β-CD and Si-MCM-41-β-CD/MG show broad bands of -OH groups at 3445 cm-1 and 3440 cm-1 respectively which may be both due to silanol as well as hydroxyl groups of βCD. -OH bending vibrational mode of silanol group appear at 1639 cm-1

[56]

. Both

asymmetric and symmetric Si-O-Si stretching modes appear at 1087 cm-1 and 801 cm-1 [57]. – C=O stretching and scissoring vibration band of –CH2 group appear at 1720 cm-1 and 1495 cm-1 respectively. The presence of N-H bending vibration at 716 cm-1 reveals the incorporation of β-CD on pure solid successfully. But, after sorption of MG, -OH bending vibrational mode of Si-OH shifted to 1632 cm-1 and asymmetric Si-O-Si stretching band moved to 1078 cm-1. Beside this, symmetric Si-O-Si stretching band moved to 790 cm-1.


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Stretching vibration of –C=O moved to 1709 cm-1 whereas –CH2 scissoring vibration moved to 1473 cm-1

[42]

. Bending vibration of N-H moved to 701 cm-1. On the otherhand, for Si-

MCM-41-PEI, -OH bending vibrational mode of silanol group appear near 1640 cm-1. Both, asymmetric Si-O-Si stretching band appear at 1084 cm-1. Symmetric Si-O-Si stretching band is observed at 799 cm-1 whereas –C=O stretching vibration appear at 1719 cm-1

[42]

. Beside

this, scissoring vibration of the –CH2 group appears at 1490 cm-1. The presence of N-H and symmetric –NH3+ bending vibration at 709 cm-1 and 1533 cm-1 confirms successful implementation of PEI on virgin material. But for Si-MCM-41-PEI/CR, -OH bending vibrational mode of silanol group shifted to 1633 cm-1. The shifting of Si-O-Si asymmetric stretching mode to 1069 cm-1 is also observed. Symmetric Si-O-Si stretching band moved to 787 cm-1 respectively. The vibration bands related to –C=O stretching and scissoring vibration moved to 1707 cm-1 and 1469 cm-1 respectively. The shifting of both N-H bending and –NH3+ bending vibrational bands took place to 700 cm-1 and 1527 cm-1 respectively [42]. Moreover, for Si-MCM-41-APA, -OH bending mode of Si-OH appears at 1635 cm-1. The band at 1082 cm-1 is because of the existence of asymmetric Si-O-Si stretching band. Symmetric Si-O-Si stretching appears at 794 cm-1. Stretching vibration of –C=O appear at 1718 cm-1. –CH2 scissoring and N-H bending vibration appears at 1482 cm-1 and 711 cm-1 respectively. All those peaks confirm the successful incorporation of APA into mesostructured calcined material. But, after adsorption of MV, -OH bending mode of Si-OH shifted to 1631 cm-1. Si-O-Si asymmetric stretching moved to 1076 cm-1. Symmetric Si-O-Si stretching modes moved to 783 cm-1. The shifting of –C=O stretching is observed at 1714 cm-1 [42]. Moreover, –CH2 scissoring and N-H bending vibration moved to 1471 cm-1 and 704 cm-1 respectively. So, from above discussions, it may be concluded that hydrogen bonding interaction may involve between groups existing on the surface of sorbents and the useful sites of dyes.



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XRD patterns (Figure 1c) of Si-MCM-41-APA, Si-MCM-41-PEI, and Si-MCM-41-β-CD show a prominent (100) peak which is attributed to the retainment of hexagonal orientation. But, a perusal of the spectra regarding Si-MCM-41-APA/MV, Si-MCM-41-PEI/CR and SiMCM-41-β-CD/MG showing absence of peaks, clearly disclose that the hexagonal characteristics of the dye adsorbed functionalised solids have been diminished, resulting in remarkable adsorption of toxic dyes. The nitrogen sorption isotherms were performed for all pre and post adsorbed materials and summarised in Figure (1d). For Si-MCM-41-APA, Si-MCM-41-PEI and Si-MCM-41-β-CD, high density of APA, PEI and β-CD on the pure material shows high surface area (755 m2/g, 678 m2/g, and 659 m2/g respectively), mean pore diameter (2.1 nm, 2.0 nm, and 1.9 nm respectively), and overall pore volume (0.7 cm3/g, 0.6 cm3/g, and 0.6 cm3/g respectively). Those isotherms retain type IV which is a common feature of MSM.[58] Furthermore, a perusal of Figure (1d) and Table 1 reveals that when dyes are adsorbed on functionalised MSMs, surface area, mean pore diameter and overall pore volume have remarkably diminished. It can be presumed that dyes may enter into the small pores of those solids for which those pores become inaccessible to nitrogen molecules during analysis. FESEM was performed for all the solids and presented in Figure 2. After functionalisations (Si-MCM-41-APA, Figure 2a; Si-MCM-41-PEI, Figure 2c; and Si-MCM-41-β-CD, Figure 2e), the pores have been partly filled with used organic modifiers. In the XRD patterns, peak intensities of those prepared sorbents have been diminished. Moreover, during nitrogen sorption analysis, nitrogen molecules cannot penetrate those pores. This indicates that particles show partially amorphous nature. Adsorption of dyes (MV on Si-MCM-41-APA, Figure 2b; CR on Si-MCM-41-PEI, Figure 2d; and MG on Si-MCM-41-β-CD, Figure 2f) leads to more amorphous nature of the particles as the functionalised pores have been sufficiently filled with those used dyes and crystalline characters of the pores have been


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drastically decreased. So, it is obvious that sorption of those used toxic dyes have taken place to a large extent on those prepared sorbents. TEM (Figure 3) was performed for all pre and post adsorbed materials. A perusal of Figure 3a, c, and e reveals the surface functionalisations (Si-MCM-41-APA, Figure 3a; Si-MCM-41PEI, Figure 3c; and Si-MCM-41-β-CD, Figure3e) were successfully implemented on the virgin MSM. After adsorption of dyes (MV on Si-MCM-41-APA, Figure 3b; CR on SiMCM-41-PEI, Figure 3d; and MG on Si-MCM-41-β-CD, Figure 3f), pores have mostly become covered (indicated by arrow). So, from the view point of all those analyses, it is experimentally demonstrated that dyes adsorption may induce some innate chaos in the pores of the surface functionalised mesostructured silica but not deformation of the structure. 3.2. Adsorption Characteristics. Dye adsorption from water using adsorbent is popular to depends upon several aspects like pH of the medium, contact (equilibrium) time, temperature of the medium, concentrations of dye medium, sorbent dose, and stirring speed.[53] The details have been given in SI (Optimization of Sorption Conditions, Figure S3-S4). Adsorption Kinetics. It is established that adsorption kinetics helps in investigating the rate as well as the mass transfer mechanism from liquid phase to solid active surface of sorbent. Uncountable kinetics models have been done to evaluate the sorption mechanism onto an adsorbent. For this purpose, pseudo-first-order,[59] pseudo-second-order,[60] second order,[61] and intraparticle diffusion[62] kinetics models were investigated. Pseudo-first-order and second order kinetics models were discussed in SI. The linear form of the pseudo-second-order equation is represented as: t 1 t = + 2 qt k2qe qe

(4)



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Where, k 2 (g mg-1 min-1) is the pseudo-second-order rate constant. Figure 4 (a,b,c) shows the plot of t/𝑞𝑡 vs. t and the values of parameters k2, qe, R2 and χ2 have already been given in Table S1-S3, SI. It can be seen that dye sorption may be best explained by pseudo-secondorder model with higher regression coefficients (R2) and lower χ2 values, compared to other models. This implies that pseudo-second-order kinetics model is appropriate to demonstrate the sorption kinetics. In otherwords, sorption kinetics depends on the amount of dye adsorbed on the active surface of sorbent and the amount of dye adsorbed at equilibrium. Moreover, the intraparticle diffusion kinetics model has been represented in Figure 4 (d,e,f) and demonstrated (linear equation) as: 1

(5)

qt = k4 t2 + c Where,

k 4 (mg g-1 min-1/2) is rate constant, c is the width of boundary layer and the

parameters k 4 , c, R2 and χ2 are described in Table S1-S3, SI. The plots are split into two straight lines indicating the involvement of dual steps in sorption process. The first part implies mass transfer of sorbate from solution to solid sorbent which is attributed to the diffusion on the boundary layer. The second part represents a balanced adsorption of sorbates throughout the surface of sorbents which is ascribed to the intraparticle diffusion[46]. Moreover, the rate limiting step involves intraparticle diffusion when the plots qt vs. t1/2 passes through the origin[46]. But, it implies boundary layer thickness when the plot does not pass through the origin. This indirectly shows the involvement of intraparticle diffusion in sorption mechanism. So, it is expected that the surface sorption and intraparticle diffusion mechanism may happen together during sorption process. Adsorption Isotherms. In adsorption, evaluation of distribution coefficient as well as distribution of solute between liquid phase and solid phase can be interpreted by incorporating different well known isotherm models namely Langmuir,[63] Freundlich,[64] and Temkin.[65] Freundlich and Temkin isotherm models have been demonstrated in SI (Figure 12 ACS Paragon Plus Environment

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S6). The Langmuir demonstrates that the monolayer adsorption happens at specific homogeneous sites, once an adsorbate incorporates in certain pores, no further adsorbate assimilates there further. Langmuir equation has been expressed in the linear form: ce 1 ce = + qe qmb qm

(6)

Where at equilibrium, the amount of adsorbed dye is expressed as q e (mg g-1), the equilibrium concentration of sorbate is indicated as ce (mg L-1), the maximum sorption efficacy is represented as q m (mg g-1) and the energy of adsorption is denoted as b. Langmuir isotherms have been given in Figure 4 (g, h, i) and parameters are represented in Table S4S6, SI. It is understandable that isotherms are linear and regression coefficients (R 2) are high, indicating the fitness with Langmuir model. In this respect, separation factor (R L ) is a salient dimensionless parameter, which can be explained by using eq. 6: RL = (

1 ) 1 + bC0

(7)

Where, the concentration of dye medium is expressed as C0 , and Langmuir constant is indicated as b. R L value suggests that if the Langmuir model is linear (R L =1), favourable (0

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